1. Field of the Invention
The present invention relates to a method for driving a charge injection device. More particularly, the invention relates to an improved method for driving a charge injection device, hereinafter referred to as a CID, which is used as an image sensing device for bi-dimensional images.
2. Description of Prior Arts
A CID is known as a solid state imaging device which reads out corresponding electrical image signals by sensing the optical radiation pattern.
A known CID has, in general, picture elements, each of which is composed of a pair of insulating electrodes, arranged bi-dimensionally on the semiconductor substrate which is covered with insulating film at the surface. Corresponding electrodes of each row are connected in common to vertical X buses and other corresponding electrodes of each column are connected in common to the horizontal Y buses. When an optical image is projected onto the sensing surface of the matrix under the condition that a voltage of about 10 V is applied to the one electrode 2X of picture elements and a voltage of about 20 V to the other electrode, the charges 4 in the semiconductor substrate generated by the optical activation are stored in the potential well 3b formed just under the electrode 2Y. Thereafter, said charges 4 are shifted at a time for each picture element to the well 3a just under the adjacent electrode 2X in such a timing that the wells just under the electrode 2Y connected to the selected Y bus are disappeared at a time by the voltage operation from the shift register 20 in the vertical direction. The condition during such process can be seen in the picture element group of 3rd stage in FIG. 1. The image charge is generated on the electrode 2X at the moment where such charge 4 is shifted, and said image charge is sequentially read out to the output terminal of the charge sensitive amplifier 7 via the electronic switches 11, 12, 13, 14. Here, 10 is the horizontal shift register; 9 is the reset switch which applies a voltage V.sub.S to the electrode 2X of each picture element; X.sub.1 to X.sub.4, Y.sub.1 to Y.sub.4 are X bus group and Y bus group, respectively.
When readout for one frame of said CID completes, the well to which the charges generated by optical activation are shifted can be made to disappear by turning ON the electronic switches, whereby the charges are injected into the substrate and disappear therein. The succeeding charges generated by optical activation are then stored in the well newly formed just under the other electrode preparing for the image sensing of the next single frame.
Here, the other electrode has the function of storing signal charges, so it is called the storing electrode, while the adjacent electrode has the function of detecting signal charges, so it is called the detecting electrode.
The peripheral circuits as well as the charge sensitive amplifier of the aforementioned CID may be integrated on the same substrate. However, the upper limit of frequency response of the charge sensitive amplifier is usually set to a frequency as low as 1 MHz, which means that it is difficult to read out the signal within a period shorter than one microsecond. It is therefore impossible to use a high speed drive having the aforedescribed known structure for a CID where a plurality of picture elements are arranged with high density.
In order to solve such problem, it is sufficient to sequentially read out in parallel all signals, in the sequence of columns, for example, sent from the picture elements arranged in a line in the vertical direction. However, it is always very difficult to inject all picture element charges into the substrate after completing parallel readout in the sequence of columns and store the optically activated charges by immediately preparing new wells. In other words, if the lifetime of charges injected into the substrate is comparatively long such as, for example, one microsecond, if the next storing operation is started while the charges injected first into the substrate have not yet disappeared and remain, such remaining charges partly enter the newly formed well drastically degrading the readout image. The partial entering of the remaining charges into the new formed well is known as the cross-talk phenomenon.
In order to solve such cross-talk problem, sufficient idle time may be prepared for the disappearance of injected charges after the stored charges of all the picture elements are injected into the substrate. This prevents the cross-talk perfectly, but results in the frame time becoming long due to the setting of idle time, thus making the realization of high speed operation difficult.
The principal object of the invention is to provide a method for driving a CID which solves the abovementioned problem caused by the injected charges.
An object of the invention is to provide a method for driving a CID which successfully solves the problem of residual image and deterioration of resolution resulting from the injected charges.
Another object of the invention is to provide a method for driving a CID which realizes high speed sequential readout without any cross-talk.
Still another object of the invention is to provide a high speed driving method for a CID which assures high quality image signals.